There are numerous driving circuits known in the prior art for energizing switched reluctance (SR) machines, such as those shown in FIGS. 1A and 1B. One commonly used circuit is comprised of an independent "half-bridge" for each phase. FIG. 2 illustrates such a circuit 100 for a two-phase machine. Circuit 100 uses two switches SA1 and SA2 and two freewheeling diodes DA1 and DA2 for phase winding A. Likewise, this circuit uses two switches SB1 and SB2 and two freewheeling diodes DB1 and DB2 for phase winding B. Phase windings A and B are respectively connected to the circuit at connections CA1, CA2 and CB1, CB2. Accordingly, 2N wire connections are needed to connect a typical drive circuit to an SR machine having N phases.
Another well known SR machine driving circuit, known as the "Oulton" (TM) driving circuit 102, is shown in FIG. 3, as configured for two phases. While driving circuit 102 requires only three connections (CN1, CN2 and CN3) for connection to an SR machine having 2 phases, Oulton driving circuit 102 requires the use of two capacitors (CB1 and CB2). The DC link voltage is split with the capacitors.
Other prior art SR machine driving circuits are described in "Switched Reluctance Motors and Their Control" (1993) by T. J. E. Miller, as well U.S. Pat. Nos. 5,075,610; 5,084,662 and 5,115,181.
For driving circuits requiring 2N connections (where N is the number of phases) there are several disadvantages. Where the driving circuit is located remote from the machine, a large number of wire connections can be costly, and result in reliability and efficiency problems. In cases where long wire leads are needed to connect the SR machine with the drive circuit (e.g., where the SR machine is to be located in a deep well) a substantial cost is incurred for the wires. Reliability is also a potential problem, since the more wire leads needed, the greater the chance for disconnection of a wire at the machine or drive circuit. Moreover, there are more opportunities for a wire to be severed along the length thereof. With regard to efficiency, potential energy loss may increase with the number of wires.
In the case of the prior art SR machine driving circuits requiring fewer than 2N connections, other drawbacks are encountered. In this regard, these driving circuits may require additional circuit elements (e.g., capacitors or inductors), require a split supply, prohibit phase overlap, or lack all three modes of operation (i.e., positive voltage loop, zero voltage loop and negative voltage loop).
In view of the foregoing, there is a need for a driving circuit which reduces the number of connections needed per phase, but does not have the drawbacks associated with prior art driving circuit designs.